1. Field of the Invention
The present invention relates to ceramic articles, and more particularly to ceramic articles having properties suitable for use in exhaust after-treatment applications, particularly diesel exhaust filtration.
2. Technical Background
Recently, much interest has been directed towards the diesel engine due to its efficiency, durability, and economical aspects. However, diesel emissions have been scrutinized both in the United States and Europe. As such, stricter environmental regulations will require diesel engines to be held to the similar standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is efficient, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
Tighter regulations continue to drive the improvement of after-treatment technologies for the abatement of diesel emissions. A 4-way emission reduction system (which simultaneously control HC, NOx, CO and PM emissions) is the future technology desired by many original equipment manufacturers. Current 4-way emission reduction technology propose use of a NOx adsorber (storage) material coated on a fine porous ceramic filter. In use, the engine cycles between rich and lean operation modes. During lean engine operation, particulate matter is oxidized by active oxygen released from the NOx adsorbers and excess oxygen from the exhaust. During the rich mode, NOx is reduced while particulate matter is also oxidized by active oxygen released during the adsorbed NOx reduction.
Cordierite is presently being proposed as a porous substrate material for NOx adsorbing emission reduction systems, however its use has been limited to only barium-based NOx adsorbing catalyst formulations. However, NOx slippage with barium-based applications is a concern, as such slippage may occur during uncontrolled exotherms exceeding 600° C. In contrast, NOx adsorber based upon a potassium (K) catalyst would be highly desired by engine manufacturers because it may offer a broader and higher operable temperature window for NOx reduction activity. Additionally, potassium (K) is also preferable as it is not regulated as toxic heavy metal that is detrimental to the environment and human health.
However, cordierite, a primary ceramic material for present after treatment systems, contains silicate phases that are prone to react with potassium-based NOx adsorber washcoats used in diesel particulate NOx reduction (DPNR) systems. To this end, silicates within current cordierite ceramic substrate technology have been shown to leach and react with potassium based catalysts to form potassium silicate. This leaching reaction compromises both the durability of the cordierite filter substrate and NOx absorption function of the potassium washcoat. For this reason, cordierite is considered to be incompatible with a potassium-based DPNR system. Further, although operating temperatures of current DPNR systems are typically below 800° C. due to the presence of the catalyst, the low heat capacity of cordierite can present additional concerns. In particular, current DPNR systems typically require a ceramic support substrate having a porosity of at least 50%. This relatively high porosity reduces the heat capacity of the cordierite substrate and decreases thermal mass, thereby increasing the likelihood of a possible temperature excursion leading to a failure during use.
Accordingly, there is a need in the exhaust after treatment art for a durable potassium-resistant ceramic article having excellent thermal shock resistance, high heat capacity, and refractory character, that is suitable for use as a catalyst support substrate in diesel exhaust gas treatment and, in particular, in 4-way and NOx diesel exhausts treatment applications.